For oxygen gas production by the PSA method, zeolite adsorbents selectively adsorbing a nitrogen gas are used and, until now, an adsorbent comprising a crystalline zeolite X or A which is ion-exchanged mainly with a calcium cation has been used.
On the other hand, it is known that a crystalline zeolite X which is ion-exchanged with a lithium cation is excellent in the separation characteristics of air as disclosed in U.S. Pat. No. 3,140,933, and recently the adsorbent comprising the lithium-exchanged crystalline zeolite X has increased.
U.S. Pat. No. 3,140,933 shows that the performance of the crystalline zeolite X is higher as the lithium-exchange ratio increases, and this fact has been re-confirmed in JP-B-5-25527 (the term "JP-B" as used herein means an ' examined published Japanese patent application) and U.S. Pat. No. 5,268,023.
The lithium-exchanged crystalline zeolite X is obtained by ion-exchanging a crystalline zeolite X using an aqueous solution containing a lithium salt. However, because lithium, which is a scarce metal, is expensive, the crystalline zeolite X adsorbent having a high lithium-exchange ratio is a very expensive adsorbent.
On the other hand, mixed cation-exchanged zeolite X adsorbents, wherein the ratio of the expensive lithium cations is reduced by mixing 5 mol % or more, substantially from 15 to 30 mol %, of alkaline earth metal cations with the lithium cations, are proposed as described in U.S. Pat. Nos. 5,174,979 and 5,152,813.
However, in the crystalline zeolite X containing 5 mol % or more of an alkaline earth metal cation, the adsorbing amount of nitrogen is large but the co-adsorption of oxygen is also large, which results in decreasing the selective adsorption property of nitrogen, whereby the performance of such a crystalline zeolite X as an adsorbent for air separation is insufficient.
Despite the fact that the crystalline zeolite X adsorbent which is ion-exchanged with lithium cations has excellent nitrogen selective adsorption characteristics, in the crystalline zeolite X adsorbent having a high lithium-exchange ratio, the cost of the adsorbent and the cost of the oxygen gas produced using such an adsorbent are very expensive.
Thus, it has been strongly desired to develop an adsorbent excellent in the cost performance that a content of expensive lithium is minimized as low as possible and which has an excellent nitrogen selective adsorption property.